**Subject :**Electrical Machines II (AC Machines)

**Unit :**Alternators

## EMF method

**EMF method:**

In this method of finding the voltage regulation of an alternator, we find the synchronous impedance Z_{s} (and hence synchronous reactance X_{s}) of the alternator from the O.C.C. and S.S.C. For this reason, it is called synchronous impedance method. The method involves the following steps:

(i) Plot the O.C.C. and S.S.C. on the same field current base as shown in Fig. (1).

(ii) Consider a field current If. The open-circuit voltage corresponding to this field current is E_{1}. The short-circuit armature current corresponding to field current If is I_{1}. On short-circuit p.d. = 0 and voltage E_{1} is being used to circulate the snort-circuit armature current I_{1} against the synchronous impedance Z_{s}. This is illustrated in Fig. (2).

Note that E_{1} is the phase value and so is I_{1}.

Fig: 1 Fig: 2

(iii) The armature resistance can be found as

(iv) Once we know R_{a} and X_{s}, the phasor diagram can be drawn for any load and any p.f. Fig. (3) shows the phasor diagram for the usual case of inductive load; the load p.f. being cos Φ lagging. Note that in drawing the phasor diagram, current I_{a} has been taken as the reference phasor. The I_{a}R_{a} drop is in phase with I_{a} while I_{a}X_{s} drop leads I_{a} by 90°. The phasor sum of V, I_{a}R_{a} and I_{a}X_{s} gives the no-load e.m.f. E_{0}.

**Drawback of EMF method:
**

This method if easy but it gives approximate results. The reason is simple. The combined effect of XL (armature leakage reactance) and X_{AR} (reactance of armature reaction) is measured on short-circuit. Since the current in this condition is almost lagging 90°, the armature reaction will provide its worst demagnetizing effect. It follows that under any normal operation at, say 0.8 or 0.9 lagging power factors will produce error in calculations. This method gives a value higher than the value obtained from an actual load test. For this reason, it is called pessimistic method.